1. Technical Field
The present invention generally relates to communications with mobile nodes in Internet Protocol (IP) networks and in particular to mobile IP control messages employed to configure communications for mobile nodes. Still more particularly, the present invention relates to the structure of extensions employed with mobile IP control messages.
2. Description of the Related Art
Explosive growth in the use of wireless or “mobile” communications devices to access Internet Protocol (IP) networks such as the Internet has lead to the development of IP mobility support, protocol enhancements which allow transparent routing of IP datagrams to mobile nodes within the Internet. These protocol enhancements support changes in the point of attachment for a mobile node from one network or subnetwork to another utilizing a home agent, a router on the mobile node's home network which maintains current location information for the mobile node and which tunnels datagrams for delivery to the mobile node when the mobile node is away from the home network, and a foreign agent, a router on a mobile node's “visited” network which provides routing services to the mobile node.
IP mobility support allows the mobile nodes, which each have a fixed “home” IP address corresponding to their home network(s), to register a “care-of” address with a foreign agent, where the care-of address is the termination point of a tunnel toward the mobile node for datagrams forwarded to the mobile node while it is away from home. Registration of the care-of address is achieved through a registration request and a registration reply, the general structures of which are illustrated in FIGS. 3A and 3B, respectively. Both the registration request and the registration reply include a fixed portion 302a and 302b followed by one or more extensions 304a and 304b. 
The extensions 304a and 304b are part of a general extension mechanism employed by mobile IP to allow optional information to be carried by mobile IP control messages. In addition to registration requests and registration replys, agent discovery control messages, such as router advertisement and router solicitation messages defined for ICMP router discovery and employed by mobile IP for agent discovery, may also include extensions. Extensions allow variable amounts of information to be carried within each datagram. Each extension is encoded in the type-length-value format illustrated in FIG. 3C, in which:    Type Indicates the particular type of extension.    Length Indicates the length (in bytes) of the data field within the corresponding extension, NOT including the Type and Length bytes. The Length field is utilized to skip the Data field in searching for the next extension.    Data The particular data associated with the corresponding extension. This field may be zero or more bytes in length. The format and length of the Data field is determined by the Type and Length fields.
The Type field in the mobile IP extension structure can support up to 255 uniquely identifiable extensions. Several types are currently defined for mobile IP control messages:
32 Mobile-Home Authentication
33 Mobile-Foreign Authentication
34 Foreign-Home Authentication
In addition, mobile IP defines the following types for extensions appearing within ICMP Router Discovery messages:
0 One-byte Padding (encoded with no Length or Data field)
16 Mobility Agent Advertisement
19 Prefix-Lengths
As large scale mobile IP deployment becomes imminent, there are many proposals for new extensions for Mobile IP, creating a strong possibility that the available type space will be exhausted and generating a real need to conserve the type field within the extensions structure.
It would be desirable, therefore, to provide a new extensions structure for mobile IP control messages which would make the extensions truly extensible and secure.